Population-averaged response to step-up and step-down stimuli. (*A*) The population-averaged response to a step up in attractant concentration, delivered at *t* = 0. Individual tumble bias traces were normalized by the mean prestimulus tumble bias before averaging across the population. Solid colored lines designate the averaged response at different stimulus levels [changing from 0 μM to 0 (control), 1, 5, 10, 50, 100, and 1,000 μM of l-aspartate, color coded in black, orange, red, purple, green, blue, and brown, respectively). Light gray lines denote one standard error above and below the mean. Black lines describe fit to a theoretical model of the chemotaxis network, with an added overshoot feature. The vertical gray band near *t* = 0 corresponds to the time when cells were moved along the chemical gradient and data was not recorded. The number of different cells (each cell was stimulated only once) included at each stimulus level: *n* = 10, 13, 22, 26, 20, 39, and 14, from top to bottom. (*B*) Same as *A*, for step-down stimuli (changing from 5, 100, and 500 μM to 0 μM of l-aspartate, color coded in red, blue, and brown, respectively). In this case, the theoretical fit (black lines) after stimulus is the sum of two exponentials. The number of different cells (each cell was stimulated only once) included at each stimulus level: *n* = 13, 15, and 14, from top to bottom. (*C*) The average adaptation time as a function of stimulus strength. Adaptation time in step-up experiments (solid black circles) was the time at which the model fit recovered to half the pre-stimulus tumble bias. Error bars are standard errors obtained from bootstrapping. The solid black line is a fit to a receptor free-energy model (). The dashed gray line is the mean of the three step-down data points (open gray circles). See *SI Materials and Methods* for more details of data analysis and modeling.

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